The present invention relates to a diagnosis system for a motor vehicle.
Recently, a motor vehicle has been equipped with an electronic control system for controlling various components of an engine, such as fuel injectors, thereby improving driveability, exhaust gas emission, fuel consumption and engine power. The electronic control system controls the components based on information represented by output signals from various sensors for detecting engine operating conditions. Accordingly, if malfunctioning of components and sensors occurs, the engine does not properly operate.
However, because of the complexity of the electronic control system, it is difficult to immediately find out the trouble. Accordingly, a trouble diagnosis device for easily checking the electronic control system should be provided in an auto shop. The electronic control system has a memory and a communication system to be connected to the trouble diagnosis device.
Japanese Patent Application Laid-Open 58-12848 discloses a diagnosis system in which a bidirectional communication system is provided between the electronic control system and the trouble diagnosis device, thereby diagnosing data based on output signals from various sensors and control data for various actuators in the control system through a single diagnosis device.
When the electronic control system is communicated with the diagnosis device and the diagnosis operation starts, the control system continuously transmits the data signal to the diagnosis device. Accordingly, the load exerted on a central processor unit (CPU) provided in the control system becomes extremely large.
If the speed of the engine increases to a high speed during the continuous data transmission from the control system to the diagnosis device, control programs of the control system such as fuel injection control and ignition timing control are delayed because of the small capacity of the CPU. As a result, the fuel injection and ignition timing can not be properly controlled.
In order to solve such problems, the continuous data transmission from the control system must be stopped by cutting the power for the diagnosis device and by cutting the power for the control system which is done by turning off a key switch of the engine.
Further, the motor vehicle has the electronic control system comprising a plurality of control units for controlling the engine, transmission, brake and cruise control. When these control units are diagnosed, the control units are connected to the diagnosis device through bus lines and connectors.
When one of the control units is in communication with the diagnosis device, the control unit continuously transmits signals to the diagnosis device. Under such a condition, another control unit is not diagnosed. In order to diagnose the other control unit, as described above, the key switch must be turned off to stop the engine to cut off the power of the control units.
Accordingly, whenever the diagnosis for one of the control units terminates, the key switch must be turned off to stop the engine. Thus, diagnosis operation for a plurality of control units can not be easily performed.